This is one of the most interesting requests I’ve ever gotten, so thank you for that!
Before I start, let me state the obvious;
Technically, there is no ‘anatomically correct’ when it comes to people with four arms - since it is not possible for humans to have it the way we represent it in art. But! That doesn’t mean there are ways to make it look anatomically believable. And this is what I will try to do.
1. We’ll start of with a simple picture of a bare back with stretched arms - as it is here you will place the four extra arms. (If you meant to only have one pair of extra arms, don’t worry, this tutorial would still be the same. As would it be with six extra arms as well.) The first tip I will give you is to draw the ‘normal’ arms first! Have them ready before you you draw the extras.
Now, let’s study how the ‘anatomy’ should work…
2. Notice how the ‘motion’ fo the back is. Those orange arrows show what direction the extra arms should be facing. This way, it’ll look more natural. Note that the shoulders, shoulder-blades and hip area are off limits to extra arms. If you place them here, it will look incredibly unnatural.
3. The best idea is to place the arms as close as possible in a similar ‘pattern’. This example shows the best areas for four extra arms. Now, if you only want two, I recommend you place them under the shoulder-blades. If it’s six, place the extras between the shoulder blades. If it’s eight, place the extras on the lower back. This way it will be the most believable. Realistically, of course, this wouldn’t work - but there are ways to trick your eyes.
4. Add the arms and… bam! There you have it; arms on the back. If you need help on how to draw arms, here’s a tutorial I’ve made on them.
5. Do some practices. My tip is is to draw each arm differently; make sure they’re all in motion. Of course, realistically, the arms would get tired after a while - but let’s not over-think it, shall we? It’s very important to draw each arm differently as it will say a lot about the character and give more life to them. If you simply copy/paste identical arms you’ll notice how much more stiff and artificial it will look. I’ll show Usui as an example;
Here you can see the arms have been copy/pasted. It doesn’t really look natural, does it? In this case this is a good thing; for Usui is not human and we can tell by the arms alone that it looks unnatural. So - both methods can be used. Just be careful with how you use them.
6. One thing you can never forget, however, is the silhouette. Make sure it all looks visually appealing as one shape, or else it will look like a confusing mess. One way to check this is to zoom out or to fill it all in black to see how it looks as a bundle.
7. Be aware of a mess when you draw several arms. It will look terribly confusing at first, but give it a chance. I suggest you be as messy as you possibly can and think of the silhouette first, then the anatomy. Don’t worry about how cleans it looks, because…
8. It will be OK in the end. <3 If you draw traditionally, make sure to sketch your arm with a gentle grip. If digitally, remember; there’s always another layer.
And that’s what I have to say about that! Hope it helped.